This invention relates generally to the art of electrochromic devices and more particularly to transparent complementary counterelectrodes.
Conventional electrochromic cells comprise a thin film of a persistent electrochromic material, i.e. a material responsive to the application of an electric field of a given polarity to change from a high-transmittance, non-absorbing state to a lower-transmittance, absorbing or reflecting state and remaining in the lower-transmittance state after the electric field is discontinued, preferably until an electric field of reversed polarity is applied to return the material to the high-transmittance state. The electrochromic film is in ion-conductive contact, preferably direct physical contact, with a layer of ion-conductive material. The ion-conductive material may be solid, liquid or gel. The electrochromic film and ion-conductive layers are disposed between two electrodes.
As a voltage is applied across the two electrodes, ions are conducted through the ion-conducting layer. When the electrode adjacent to the electrochromic film is the cathode, application of an electric field causes darkening of the film. Reversing the polarity causes reversal of the electrochromic properties, and the film reverts to its high transmittance state. Typically, the electrochromic film, e.g. tungsten oxide, is deposited on a glass substrate coated with an electroconductive film such as tin oxide to form one electrode. The counter electrode of the prior art has typically been a carbon-paper structure backed by a similar tin oxide coated glass substrate or a metal plate.
While this conventional electrochromic device structure might be acceptable for data displays in items such as digital watches, it is not suitable for large transparent articles such as windows. While the opaque carbon-paper counter electrode may be replaced with a thin conductive film such as tin oxide, indium oxide or gold, these thin film electrodes encounter lateral electrical resistance which decreases the speed and uniformity of charge distribution as the surface area of the device increases. More importantly, with electric fields of about 1 volt, half-cell reactions which result in the evolution of gas from the electrolysis of water occur at the counter electrode, depending on the polarity, as follows:
______________________________________ Electrode Reaction Standard Potential ______________________________________ Cathode 2H.sub.2 O + 2e.sup.- - H.sub.2 + 2OH.sup.- -0.828 volts Anode 2H.sub.2 O - 4H.sup.+ + O.sub.2 + 4e.sup.- -1.229 volts ______________________________________
The hydrogen and oxygen gases produced by these reactions form bubbles which impair the optical properties of an electrochromic cell for use as a window.
The use of a metal mesh as the counter electrode is described in U.S. Pat. No. 4,768,865, the disclosure of which is incorporated herein by reference. The invention described therein allows transparency while providing uniform rapid charge distribution over a large surface area and participating in a balancing half-cell reaction at a lower potential which prevents electrolysis of water and concurrent gas evolution which would otherwise occur according to the following reactions, wherein x is typically up to about 0.5: ##STR1##
Instead of the hydrolysis of water at the counter electrode, pictured on the right above, the balancing half-cell reaction in response to the electrochromic transition of tungsten oxide is the oxidation or reduction of the metal of the metal grid counter electrode, which does not produce gas which can form bubbles and decrease the optical quality of the device.
European Patent Application No. 84303965.2 of Ataka et al. discloses a secondary cell comprising an anode, a cathode and an electrolyte in contact with both. The cathode contains as an electrochemically active material an oxidation product of a polynuclear metal cyanide complex of the general formula EQU [M.sup.A ].sub.K [M.sup.B (CN).sub.6 ].sub.L .multidot.X H.sub.2 O
where M.sup.A is a metal having a valence of A, M.sup.B is a metal having a valence of B, K is from 1 to 4, L is from 1 to 3 and X is a positive number including 0, and wherein A.times.K=(6-B).times.L. The anode contains as an electrochemically active material a reduction product of the polynuclear metal cyanide complex.
In the Journal of Electroanalytical Chemistry, 252 (1988) p. 461-466, Kulesza et al. describe the preparation, characterization and application of mixed-valence compounds due to their electrical conductivity and spectroscopic properties, particularly polynuclear transition-metal hexacyanometallates of the general formula [M.sup.A ].sub.x [M.sup.B (CN).sub.6 ].sub.y where M.sup.A and M.sup.B are transition metals with different oxidation states and x and y are stoichiometric subscripts. This reference describes an electrochemical method of fabricating films of indium (III)-hexacyanoferrate (III, II).
In Acc. Chem. Res., 19 pp. 162-168 (1986), Neff et al. disclose that the potassium cation is the best choice as a counter ion for Prussian blue in aqueous electrolytes because of the optimum size of its hydrated ionic radius.
In Electrochimica Acta, 34 pp. 963-968 (1989) and 35 pp. 1057-1060 (1990), Dong et al. describe the preparation of thin films of indium hexacyanoferrate(II) on conductive substrates by cyclic voltammetry. It is further disclosed in aqueous cyclic voltammetry studies that both sodium and potassium ions may be readily transported into and out of the film by application, respectively, of a cathodic or anodic potential. It is also reported that in aqueous solutions containing H.sup.+, Li.sup.+, Ca.sup.+2 and NH.sub.4.sup.+ salts, the indium hexacyanoferrate(II) films do not exhibit the redox peak characteristic of the Fe(CN).sub.6.sup.-3 /Fe(CN).sub.6.sup.-4 redox couple.
In the Journal of Physical Chemistry, 86, pp. 4361-4368, (1982), Rajan et al. disclose films of ruthenium purple, KFeRu(CN).sub.6 or NH.sub.4 FeRu(CN).sub.6 that are electrochromic analogous to reduction of Prussian blue, but with no corresponding oxidation to the analogue Berlin green.
In the Journal of the American Chemical Society, 104, pp. 3751-3752, (1982), Itaya et al. describe electrochemical preparation of a Prussian blue analogue, iron-ruthenium cyanide.
In the Journal of Materials Science Letters, 5, pp. 231-236 (1986), Reitman described some physical and chemical properties of anhydrous rare earth (RE) hexacyanoferrates (II and III) in the series (RE)Fe(CN).sub.6 and (RE).sub.4 [Fe(CN).sub.6 ].sub.3, excluding promethium.